A mixer (frequency mixer) has a frequency conversion function, and is an important constituent element in circuits for wireless communication. For example, high-frequency signals received by an antenna are converted into low-frequency signals by a mixer circuit. In the case of a direct-conversion receiver, for example, a mixer circuit performs frequency conversion of high-frequency signals to the baseband level. A mixer circuit multiplies input high-frequency signals by local frequency signals, and outputs the down-converted low-frequency signals and up-converted high-frequency signals. And, either the low-frequency signals or the high-frequency signals are used in a later-stage circuit.
In electronic communication fields in recent years, demands for reduced power consumption, smaller device sizes, and lower prices have led to the realization of such mixer circuits as circuits comprising MOS transistors.
A mixer circuit comprising MOS transistors has the problem that output signals are combined with 1/f noise (flicker noise). The lower the frequency, the higher is the level of this 1/f noise, so that a down-converted low-frequency signal has a lower (worse) S/N ratio at lower frequencies, causing degradation of the reception sensitivity of the receiver.
1/f noise is inversely proportional to the ½-power of the gate area of a MOS transistor (1/f noise∝1/√(gate area)). That is, the larger the gate area of a MOS transistor, the weaker the 1/f noise tends to be. However, if the gate area is made too large, the chip area increases, and the increased gate capacitance causes a reduction in operation speed and increased current consumption. Hence there is the problem that if transistor sizes are reduced, 1/f noise occurs.
As one method of suppressing 1/f noise, the current of the current source on the input side of a mixer circuit may be detected, and feedback control is executed so that the detected current equals a target value, to reduce 1/f noise. Another method is to use P-channel MOS transistors as the MOS transistors comprised by high-frequency circuits and mixer circuits to reduce 1/f noise.
The Japanese Laid-Open Patent 2007-6493 and the International Laid-Open Patent WO2003/003561 disclose a frequency mixer circuit.
The method in which the current of the current source on the input side of the mixer circuit is detected, and feedback is used such that the detected current equals a target value, is applied to specific input circuits of mixer circuits, but entails a complex circuit configuration and is not preferred.
The method in which the circuit is configured using P-channel MOS transistors requires modifications, including modifications to semiconductor device processes, and so is not preferred.